Because a tire has a geometry exhibiting symmetry of revolution about an axis of rotation, its geometry can be described in a meridian plane containing its axis of rotation. In a given meridian plane, the radial, axial and circumferential directions refer respectively to the directions perpendicular to the axis of rotation, parallel to the axis of rotation and perpendicular to the meridian plane. In what follows, the expressions “radially inner” and “radially outer” respectively mean “closer to the axis of rotation in the radial direction” and “further from the axis of rotation in the radial direction”. The expressions “axially inner” and “axially outer” respectively mean “closer to the equatorial plane in the axial direction” and “further away from the equatorial plane in the axial direction”, the equatorial plane being the plane perpendicular to the axis of rotation and passing through the middle of the tread.
A radial tire comprises a reinforcement comprising a crown reinforcement, radially on the inside of the tread, and a carcass reinforcement radially on the inside of the crown reinforcement.
The carcass reinforcement of a radial tire comprises at least one carcass layer. A carcass layer is made up of reinforcing elements, or reinforcers, coated in an elastomeric material, which are parallel to one another and make an angle substantially equal to 90°, namely of between 85° and 95°, with the circumferential direction. A carcass layer generally comprises a main part, connecting the two beads together and wound, within each bead, around a circumferential reinforcing element usually made of metal called a bead wire to form a turnup. The meridian section of a bead wire, namely the section through the bead wire on a meridian plane, is circular or inscribed inside a circle, of which the centre is commonly referred to as the centre of the bead wire. The bead wire is usually surrounded by at least one coating material such as, nonexhaustively, an elastomeric material or a textile material, the assembly comprising the bead wire and its coating material constituting a bead wire core. The carcass layer turnup allows the carcass to be anchored in each bead to the bead wire core.
It is also known practice to have, within each bead, an additional reinforcement comprising at least one additional layer. An additional layer comprises reinforcing elements, or reinforcers, coated in an elastomeric material, parallel to one another and making with the circumferential direction an angle generally at most equal to 45° and typically substantially equal to 22°. The additional reinforcement extends radially between its two, radially inner and radially outer, ends and axially on the outside of the axially outermost turnup. It is the radially inner end of the radially innermost additional layer that is referred to as the radially inner end of the additional reinforcement. Similarly, it is the radially outer end of the radially outermost additional layer that is referred to as the radially outer end of the additional reinforcement. The radial distance between the respectively radially inner and radially outer ends of the additional reinforcement is generally at most equal to 0.3 times the design section height of the tire as defined by the “European Tire and Rim Technical Organisation” or ETRTO standard. The additional reinforcement makes it possible to limit, in the bead, the deradialization of the radial carcass reinforcement, namely the variation in angle of the reinforcers with respect to their initial, substantially 90° angle as a result of circumferential deformations of the portions of elastomeric compound comprised between the reinforcers: this limiting of the deradialization contributes to the durability of the bead.
The carcass layer and additional layer reinforcers are generally either metal cords or reinforcers made up of assemblies of textile filaments, preferably of the aliphatic polyamide or aromatic polyamide type. In the case of reinforcers made up of assemblies of textile filaments, the carcass reinforcement usually comprises several carcass layers, the number of which is determined according to the level of mechanical strength that the carcass reinforcement is required to have.
As far as the rim on which the tire is mounted is concerned, the rim part intended to come into contact with a bead of a tire comprises a substantially circular portion extended radially towards the inside by a substantially radial portion constituting the rim flange intended to come into contact with the axially outer face of the bead and to fix the axial position of the said bead when the tire is mounted and inflated. It also comprises a substantially axial portion or rim seat, extending axially towards the inside from the radially inner end of the rim flange and intended to come into contact with the radially inner face of the bead. A tire according to the invention is intended to be mounted on a rim the seat of which makes an angle of 5° with the axial direction.
During running, the beads of the tire wrap around the substantially circular portions of the rim flanges and are subjected to bending cycles. These bending cycles lead to variations in curvature combined with variations in tension in the carcass layer and additional layer reinforcers. In addition, these bending cycles lead, in the elastomeric materials present in the immediate vicinity of the free ends of the carcass turnup reinforcers and radially outer ends of the additional layer reinforcers, to compressive and tensile forces which generate thermomechanical stresses and strains liable to cause cracks the spread of which could degrade the tire requiring it to be replaced.
Document FR 2055988 has already described, in the case of a radial carcass reinforcement, beads the durability of which is improved with a view to lengthening the life of the tire. Each bead comprises an additional circumferential reinforcement comprising one or more additional layers. An additional layer, substantially parallel to the carcass layers and adjacent to a carcass turnup or to a carcass layer main part comprises metal or textile reinforcers oriented substantially in the circumferential direction, which is where it gets its name of additional circumferential reinforcement. The substantially circumferential reinforcers have the advantage of not having radially outer ends likely to generate local cracks that cause the bead to start to degrade. The circumferential reinforcers may be positioned at a constant or variable spacing so as to optimize the tensile stiffness of the additional layer.